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So, Trav, you're saying I should look for Iridium plugs for the Packard then?
If that Packard can run on no lead fuel then absolutely 100% i would run them or a double platinum plug.
Sometimes it takes a while but if you can find a modern plug number for the engine a precious metal equivalent can usually be found, even for many real old timers.
I have a friend who runs them in his 55 Caribbean and it runs much smoother and starts much easier.
There are zero advantages to a traditional copper plug other than its initial lower price tag. Even if you put no value on your time at all they are more expensive in the long run based solely on their shorter life span.
Another issue that seems to go unnoticed is the fact that due to the shorter lifespan of traditional plugs the plug wires on many engines also need to be removed more often along with the changing of the plugs.
Anyone who as turned a wrench longer than a couple of years knows the issues that can cause, loose fitting or even worse unseen separation at the terminal.
The precious metal plugs (especially the double platinum) have a life span long enough that i usually just toss the wires with the plugs.
The core of these plugs is also copper just the firing ends are precious metal plated.
Their resistance to gap erosion means the firing tip can be much finer which translates into reduced firing voltage, higher resistance to fouling. etc, etc.
These qualities are welcome in engines that use older point or magneto ignitions with lower voltage coils, were and tear on ignition components is greatly reduced.
Easier starting, less fouling, smoother idle are also traits of these plugs in addition to the qualities of a "copper" plug.
Page 2 maybe of interest to you.
http://www.ngksparkplugs.com/pdf/dyk_5points.pdf